Etymological oddity
Sometimes you find words that seem like they must be related, and then
it turns out to be a complete coincidence.

Consider pen and pencil.

Pen is from French penne, a long feather or quill pen,
akin to Italian penne (the hollow, ribbed pasta), and
ultimately to the word feather itself.

Pencil is from French pincel, a paintbrush, from Latin
peniculus, also a brush, from penis, a tail, which is
also the source of the English word penis.

A couple of weeks ago someone edited the Wikipedia article on
"false cognates" to point out that day and diary are
not cognate. "No way," I said, "it's some dumbass putting dumbassery
into Wikipedia again." But when I checked the big dictionary, I found that it was
true. They are totally unrelated. Diary is akin to Spanish
dia, Latin dies, and other similar words, as one would
expect. Day, however, is "In no way related to
L. dies..." and is akin to Sanskrit dah = "to burn",
Lithuania sagas = "hot season", and so forth.